When protective film-forming finishes including paints, lacquers, and varnishes are applied to wood having a high content of water-soluble tannin, such as redwood, cedar or mahogany, the tannin often bleeds or migrates into the surface film, thereby causing a dark stain in the film. When additional coats of a protective film are applied, the stain often bleeds through and continues to be visible. Thus, tannins provide excellent anti-microbial properties in many leaf, bark and root materials but simultaneously cause undesirable staining problems in film-forming wood finishes.
Various processes of inhibiting tannin migration in tannin-containing wood substrates are known. Sulfonium compounds have been disclosed as useful in blocking tannin migration in wood substrates (U.S. Pat. Nos. 3,636,052; 3,660,431 and 3,900,619). Highly cross-linked ion exchange resins have been incorporated into aqueous coating compositions applied to cedar and redwood type substrates (U.S. Pat. No. 3,494,878). Aqueous solutions of nonlinear polyalkylenimines or linear, partially deacylated poly(N-acyl)alkylenimines have also been used to inhibit tannin migration in wood substrates (U.S. Pat. Nos. 4,075,394 and 4,104,228 respectively). Amphoteric compounds of alumina, titania, zirconium, silica or zinc have been incorporated into certain composite pigments to inhibit the migration of water-soluble tannins through a paint film (U.S. Pat. No. 3,852,087).
The lumber industry, concerned with the problem of transit staining on lumber, is taught to use an essentially atinctorial (i.e., neutral with respect to color) slowly leachable alkaline earth metal base which include oxides, carbonates, phosphates, hydroxides, borates, borosilicates, and hydrates of the alkaline earth metals, magnesium, calcium, and barium (U.S. Pat. Nos. 3,756,850 and 3,808,037). These patents do not discuss the inhibition of tannin migration through a film-forming finish on the wood substrate. The atinctorial deposit of a particulate base is used to maintain the freshly cut appearance of the lumber by maintaining the pH of the exposed surface of the lumber above about 10 for the duration of exposure to the atmosphere.
The paint, varnish and lacquer industry is still seeking an ecologically safe, effective stain-controlling agent for use under or in film-forming finishes. The variety of materials offered for the purpose is, to some extent, evidence that none is without disadvantage. Some of the materials identified by the prior art are very effective in complexing with tannins and preventing staining of top coats, but they cause discolored or tan primer coats. Thus it is also desirable to find an additive which can be incorporated into either the primer or top coats without discoloration.
As a consequence of its ability to complex with the natural dyes (i.e., tannins) magnesium hydroxide pigment used as an extender in primer coats, can prevent the staining of finish coats applied over tannin-containing wood substrates. Magnesium hydroxide is ecologically safe and does not cause discoloration in the primer coat.